More from Chicago Manual of Style's July Q&A

https://www.chicagomanualofstyle.org/qanda/latest.html

Q. “. . . go to high school in Washington[, D.C.].” Is the final period necessary? Delete it? (Yes, it’s the last of a longer quotation.)

A. In your version, where you’re using brackets to supply not just the abbreviation but the comma that would normally go with it, you don’t need that final period; we can assume that your bracketed interpolation includes all sentence punctuation, including any final period. And that’s what we might expect if you were supplying the end of a sentence that’s missing or illegible in the source. In other words, your brackets restore the end of a sentence that would normally be punctuated like this:

“. . . go to high school in Washington, D.C.”

But if you’re simply clarifying for readers that the text is referring to the district rather than the state, don’t add that comma. Instead, put “D.C.” in brackets and add the sentence-ending period:

“. . . go to high school in Washington [D.C.].”

That extra period is needed for the same reason you’d add a period to the end of a sentence like this one (from CMOS 6.13):

His chilly demeanor gave him an affinity for the noble gases (helium, neon, etc.).

But there would be no periods in an initialism like DC in current Chicago style, so you’d normally write this:

“. . . go to high school in Washington [DC].”

See also CMOS 6.110 (which has a similar set of examples but without periods) and 12.70–74 (on editorial interpolations and clarifications).

This one has been driving me crazy!

Abbreviations

Q. Is there any chance that “am” and “pm” will become acceptable as correct forms of “a.m.” and “p.m.”?

A. There are six ways to write the abbreviations for ante meridiem (before noon) and post meridiem (after noon):

All caps with periods: 10 A.M., 10 P.M.
All caps without periods: 10 AM, 10 PM
Small caps with periods: 10 A.M., 10 P.M.
Small caps without periods: 10 AM, 10 PM
Lowercase with periods: 10 a.m., 10 p.m.
Lowercase without periods: 10 am, 10 pm

Each of these—including “am” and “pm”—is a legitimate choice. For nearly a century, Chicago’s preferred form was the third: small capital letters with periods. This preference, however, applied only to published documents (among other factors, small capitals weren’t an option on typewriters).

When we changed our preference to “a.m.” and “p.m.” (in 2003, with the publication of CMOS 15), the growth of computers in writing and publishing played a role: small caps require extra steps to apply, and they don’t always translate well across applications (when they’re even available). We could have flipped a coin and settled on all-caps “AM” and “PM” (but not “A.M.” and “P.M.”; Chicago style now omits periods in abbreviations that include two or more capital letters). When we instead chose lowercase “a.m.” and “p.m.,” we liked the fact that they’re unambiguous (“AM” and “PM” both have a number of other meanings), and we hoped the periods would help readers recognize in any context that these are abbreviations, not words.

But if you don’t like the periods, don’t fret: Merriam-Webster labels “am” and “pm” as British variants, so you’re hardly alone in your preference. If you’re being published, however, be prepared to defer to your publisher’s house style, whatever that may be.